1. Field of the Invention
The present invention relates to a high-voltage generator employing a piezoelectric transformer, and particularly, to a high-voltage generator that fully utilizes the advantage of the piezoelectric transformer that it is small, thin and light, to reduce the size, thickness and weight of the high-voltage generator.
2. Description of the Prior Art
A piezoelectric transformer causes electrostriction effect and piezoelectric effect to produce a high voltage in response to an input voltage that meets the resonance frequency of the transformer.
FIG. 1 schematically shows such a piezoelectric transformer. This transformer is made of a thin ceramic plate 27. Half of each face 28 of the ceramic plate 27 is coated with an electrode 30, and an end face of the ceramic plate 27 is coated with an electrode 31. This piezoelectric transformer is small, thin, and light.
FIG. 2 shows a conventional high-voltage generator employing the piezoelectric transformer of FIG. 1. The generator includes a self-exciting oscillator that oscillates in synchronism with the resonance frequency of the piezoelectric transformer, to drive the transformer.
In FIG. 2, the high-voltage generator has the piezoelectric transformer 21, a winding transformer 22, a phase detector 23, an amplifier 24 formed of a transistor, and a load resistor 25. The winding transformer 22, phase detector 23, and amplifier 24 form the self-exciting oscillator. The piezoelectric transformer 21 is inserted in a feedback loop of the oscillator. The winding transformer 22 and phase detector 23 detect the phases of input voltage and current to the transformer 21.
The oscillator always oscillates at a resonance frequency at which the phase difference of the input voltage and current to the piezoelectric transformer 21 becomes zero. Namely, the oscillator oscillates to drive the piezoelectric transformer 21 at a frequency at which internal resistance is minimized and a feedback quantity is maximized. An output terminal of the piezoelectric transformer 21 provides an alternating-current (AC) high voltage to the load resistor 25.
FIG. 3 shows a concrete example of the conventional high-voltage generator of FIG. 2. A piezoelectric transformer 21 is inserted in a feedback loop and is driven with zero phase difference. An output of the piezoelectric transformer 21 is rectified by a rectifier 26 to supply a direct-current (DC) high voltage to a load.
In the conventional high-voltage generator, the winding transformer for detecting a phase in the self-exciting oscillator occupies a large space, to increase the overall size of the high-voltage generator. This results in deteriorating the advantage of the piezoelectric transformer that it is small, thin, and light.